In recent years, an electrophotographic system image forming method has been utilized as an alternative to preparation of original images or a so-called printing rather than simple copying, with the developments in network such as easy transmission of specifically data in a digital mode, and its application range has been expanded. Consequently, in this technique, formation of a stable image for a long duration is demanded, and longer frequency to change a developer is also demanded. On the other hand, in order to form a stable image in the electrophotographic system image forming method, a two-component developer composed of a carrier and a toner is preferably to be utilized. It would appear that this can reliably provide charge to the toner since the carrier exhibits charge providing performance, and further, quick charge-rising can be obtained since many frictional electrification-providing regions are present, resulting in suitability for high-speed development.
However, when using a two-component developer in the electrophotographic system image forming method, there appear problems such that degradation of the carrier is produced via attachment of toner components to the carrier, and external additive particles embedded in the toner are produced via repeated collision of the carrier with the toner.
In order to solve this problem, it is proposed that stress to the toner is reduced by employing a so-called weight-saved resin dispersion type carrier as a carrier (refer to Patent Documents 1 and 2). This resin dispersion type carrier is one in which low specific gravity is realized by dispersing magnetic particles in a binder resin, and specifically, since a resin dispersion type carrier prepared by a polymerization method is formed in spherical shape, and has reduced protrusions, stress to the toner is effectively reduced. And, among the resin dispersion type carriers obtained via the polymerization method, specifically, a carrier, with which a phenol formaldehyde resin exhibiting advantageous properties such as high hardness and excellent durability is used as a binder resin, tends to be preferably utilized.
However, even though using the resin dispersion type carrier employing such the phenol formaldehyde resin, there is another problem such that degradation of development is still generated when using the carrier for a long duration.
(Patent Document 1) Japanese Patent O.P.I. Publication No. 2002-214842.
(Patent Document 2) Japanese Patent O.P.I. Publication No. 2005-352473.